The present invention relates to a braking apparatus for braking a rotary unit by bringing a press unit on an actuator into contact,with the rotary unit and a method of controlling the braking apparatus.
A brake fluid pressure control system intended for anti-skid control or traction control is disclosed in JP-A-5-147524, in which the brake cylinder pressure is directly changed by operating a pump and the fluid pressure of the brake cylinder is controlled to a level not directly related to the operating condition of the brake pedal. In such a system, a gear pump is interposed between a brake master cylinder and a brake cylinder, the pump is normally in idle runing not to prevent the supply of the fluid pressure of the master cylinder, so that in the case where the pressure reduction is required as at the time of anti-skid control, the motor of the gear pump is driven to return the brake fluid to the master cylinder, while at the time of pressure increase, the motor is driven reversely to supply the brake fluid to the brake cylinder.
Another braking system comprising a brake pedal and a brake cylinder not directly connected to each other is disclosed in JP-A-10-203338, in which the brake cylinder pressure is changed directly by the operation of a pump. This system comprises a bypass circuit in parallel to the pump for supplying the brake fluid to the brake cylinder from a tank, and the brake fluid pressure is easily controlled by adjusting the opening of a variable orifice arranged midway of the circuit. Specifically, when adjusting the fluid pressure in the brake cylinder, the opening of the variable orifice but not the pump revolution speed or torque is increased thereby to reduce the fluid pressure for a smaller opening degree, thus controlling the fluid pressure upward.
In any one of the conventional systems described above, the pump or the variable orifice is controlled so that the brake fluid pressure coincides with a pressure corresponding to the required braking force.
In the normal braking apparatus utilizing the friction, however, a gap (pad clearance) is provided between a rotary member such as a brake disk and friction members such as brake pads to prevent the dragging of the friction member when the braking force is not required. As long as the brake pads are moving over the distance corresponding to the pad clearance, however, the braking force is not obtained and the vehicle runs free. Therefore, it is necessary to bring the brake pads into contact with the brake disk as early as possible. Also, the braking force is determined by the pressure exerted by the brake pads, i.e. the braking fluid pressure in the brake cylinder in the case of the fluid pressure type. For controlling the braking force, therefore, the fluid pressure is required to be controlled accurately.
According to the brake control system disclosed in JP-A-5-147524, the oil pressure supplied to the brake cylinder is generated directly by the pump, for example, the piston can move freely while the brake pads are moving over the distance corresponding to the pad clearance. Thus, the piston continues to move while generating the pressure due to the sliding resistance and the fluid resistance with the brake cylinder until the brake fluid in an amount corresponding to the pad clearance is completely supplied. From the very instant the brake pads come into contact with the brake disk, however, the piston position is restricted, and the flow rate of the brake fluid, which is. considered a non-compressive liquid, into the brake cylinder is reduced to zero. As a result, the brake fluid pressure increases sharply due to the force of inertia of the pump, the motor and the brake fluid and the delayed fluid pressure control. The degree of this sharp increase in fluid pressure depends on the flow rate, i.e. the motor revolution speed at the moment when the brake pads come into contact with the brake disk, leading to the problem that the higher the motor speed, the shorter the time when the vehicle runs free while the fluid pressure increases more sharply.
In the case where the pressure is controlled by a variable orifice as shown in the other prior art JP-A-10-203338, in contrast, the brake fluid pressure is not increased sharply but the piston travel speed changes with a pressure command value. When a small braking force is required, therefore, the pressure command value is small and the time of free run lengthens correspondingly.
The object of the present invention is to provide a braking apparatus high in responsiveness with the braking force controllable with high accuracy, and a method of controlling the braking apparatus.
In order to achieve the object described above, according to the present invention, there is provided a braking apparatus comprising a first contact unit arranged on a rotary shaft of a wheel, a second contact unit brought into contact with the first contact unit for generating the friction force, an actuator for pressing the second contact unit against the first contact unit, and a control unit for controlling the operation of the actuator, wherein the control unit performs the position control operation for moving the second contact unit to such an extent that the interval between the first and second contact units is smaller than a preset distance (clearance) and the braking force control operation for controlling the pressure exerted by the second contact unit against the first contact unit.
In the position control operation, the second friction unit is desirably set closer to the first friction unit by an amount corresponding to the preset clearance.
According to this method, the second control unit is set closer to the first contact unit by the position control operation, and the force of pressing the second contact unit against the first contact unit is controlled by the braking force control operation. Thus, the delay time of generation of the braking force is shortened and the accuracy of the braking force control operation is improved at the same time.